1. Field of the Invention
This invention relates in general to processes for obtaining cellulosic fibers from plant materials. More specifically, this invention relates to processes for removing cellulosic fibers from seed hulls such as cottonseed hulls. This invention also relates to the recovery or production of other hull commodities including xylan, xylose and xylitol.
1. Summary of the Prior Art
Seed hulls such as cottonseed hulls contain various cellulosic fibers either attached to or substantially embedded in the epidermal layer of the seed hull. The long fibers or hairs are called lint fibers or staple cotton. These long fleecy fibers or hairs are removed in cotton ginning operations and are widely used for spinning into cotton fabrics. The short fibers, not suitable for spinning, are called linters. These cotton linters, adhering to the hull of the cottonseed after ginning, are mature fibers, with increased secondary wall growth, and are just as pure a form of cellulose as staple fibers. In a cottonseed oil mill, the cottonseeds adhering linters are typically delinted so as to separate the linters from the seed hull. The delinted seeds are then decorticated or dehulled to recover the meat or kernel. Such delinting prior to dehulling has been found typically necessary so that the linters would not interfere with recovery of the meat or oil and contribute to losses in yield at the oil mill.
The linters thus obtained are useful in the manufacture of rayon, cellulosic plastics, photographic film, explosives, lacquers, cellulosic derivatives, cotton-felt, paper products, upholstery, padding, and mattress materials. For all uses the linters receive substantial secondary processing due to trash and dirt content, which may amount to as much as 25 percent of the linter material from the oil mill. Typical processing steps for linters include wet and dry mechanical cleaning, pressure digestion in caustic soda, bleaching, washing, centrifugal cleaning, and drying.
Seed hulls also contain "hull fibers" -- i.e., cellulosic fibers which are substantially embedded within the seed hull.
Heretofore, mechanical delinting operations typically removed about 80 percent of the linters adhering to the cottonseed hull. Mechanical delinting operations are not successful in removing the hull fibers from seed hulls.
Various methods have been suggested for removing cellulosic fibers from cottonseed hulls. These methods include mechanical, chemical (see, e.g., U.S. Pat. Nos. 105,923 and 159,455), and combined chemical/mechanical (see, e.g., U.S. Pat. Nos. 357,089, 1,295,078 and 1,987,195) methods.
In a typical mechanical operation, cottonseed hulls, for example, which have been separated from the meal or kernel are passed through an attrition mill which comprises a gin with saw-teeth that cuts the linters close to the seed or alternatively cuts the linters from the hulls which are incidentally fragmented and ground up. Although such processes are advantageous in that they tend to separate the linters from the seed hull components, they also have several disadvantages. For example, such mechanical delinting operations tend to produce a large amount of noise and dust resulting in potentially hazardous working conditions. Also, the linters produced in this operation tend to be shortened by the cutting action (the "foot" and "shank" of the fiber remain embedded in the hull) and are thus not useful in applications which require longer fibers.
Mechanical treatment of the cottonseed hull also results in attack upon the fiber. For example, mechanical grinding operations tend to break down the cellulosic fibers into a mesh of cellulose molecules. Also, in a saw-type mechanical delinting operation, cutting action in the delinting machine tends to shorten all the fibers, shear off hull epidermal material, and the linters recovered require extensive processing (such as washing, centrifugal cleaning and drying) before being suitable for any end use.
A typical chemical method for removing cottonseed linters from cottonseed hulls is disclosed in U.S. Pat. No. 159,455 (Rock). This patent describes a combined steam/wet alkali solution process for separating the fiber (which was not previously removed by the ginning operation) from cottonseed hulls. However, the use of alkaline solutions, either with or without the presence of steam, has several disadvantages. For example, sufficiently weak alkaline solutions do not completely dissolve the binding components in the seed hulls so as to facilitate separation of the hull residues from the fibrous component. On the other hand, alkali which is strong enough to dissolve the non-fibrous hull components at high temperature is also strong enough to damage the linters present in the cottonseed hulls, thus making them unsuitable or only poorly suited for certain applications, such as papermaking, upholstery, padding and mattress manufacture, or other textile-like application. Furthermore, strong alkali can mercerize the fibers, making them unsuitable or only poorly suited for some chemical processing applications. Besides being advantageous from a product standpoint, the use of such large amounts of alkali (i.e., that which is sufficient to dissolve the non-fibrous hull components) is economically disadvantageous from a process standpoint.
Furthermore, prior art chemical delinting processes result in a relatively low yield of cellulosic fibers and the cellulosic fibers produced are not readily washable because of high residual alkali concentration at the end of the chemical treatment stage.
Combined chemical/mechanical delinting processes have many of the disadvantages of both the chemical and mechanical delinting processes discussed above, as well as the additional disadvantage of chemically loosened fibers forming rope-like masses of significant strength, thus contributing to clogging or otherwise interferring with the mechanical beater bars or knives. Furthermore, all of the above processes result in a low yield of cellulosic fibers and the cellulosic fibers produced are not readily washable.
Other methods for treating seed hulls by chemical or combined chemical/mechanical processes are also known. For example, the non-fibrous hull components (such as polyphenolic materials) of seed hulls may be dissolved by treating the hulls in a typical soda digestion process. However, such processes suffer from the disadvantages that (1) the non-fibrous hull materials are not completely removed, (2) large amounts of alkali must be used, and (3) the large amounts of alkali required result in the fibers being difficult to wash.
The residual cottonseed hull consists primarily of cellulose, lignin, polyphenols, hemicellulose and some minor components. The hemicellulose fraction is primarily xylan which is of commercial interest for the production of various conversion products. It has been shown in the past that the xylan in cottonseed hulls can be isolated for conversion into particularly furfural, xylose, and xylitol. Prior art (U.S. Pat. Nos. 3,565,687 and 3,558,725) teaches that the xylan may be converted sequentially to xylose and xylitol without first isolating the xylan. These processes involve treating the cottonseed hulls with dilute alkali solution to remove some color-forming impurities, and then hydrolyzing the xylan-containing hull with dilute sulfuric acid to produce a xylose solution. This xylose solution, however, still contains color-forming impurities which must be removed using complex and costly ion-exchange treatments.
Accordingly, the search has continued for improved processes for obtaining cellulosic fibers from seed hulls, as well as for obtaining other seed hull commodities including xylose and xylitol. The present invention was developed as a result of this search.